As a method for controlling the charging of a secondary battery (rechargeable battery) mounted on a vehicle, it is known to maintain a state of charge of the secondary battery to a prescribed target value smaller than the charge capacity of the secondary battery. See Patent Document 1. According to this prior invention, because the energy regenerated from braking cannot be efficiently utilized if the secondary battery is always fully charged to the charge capacity, a certain margin is left in the secondary battery so that the electric energy produced by a regenerative braking may be charged to the secondary battery for efficient utilization of the electric energy. By using the charged or stored electric energy as an auxiliary drive power, the fuel economy of the engine can be improved.
In recent years, vehicles are often provided with an idle stop function which automatically stops the engine when the vehicle has come to a stop, and automatically restarts the engine when the vehicle is to start off. In an idle stop vehicle, because the battery is required to supply the electric power for the vehicle when the engine is not running, and for restarting the engine, the battery is required to be charged and discharged very frequently. Therefore, various forms of charging control devices have been developed for the purpose of properly managing the battery of idle stop vehicles. In a known charging control device of this type, the battery is charged whenever the engine is restarted in an idle stop operation. See Patent Document 2. In the charging control device disclosed in Patent Document 2, the battery is charged every time an idle stop operation is performed (restarting the engine), and the charging of the battery is terminated when the battery is fully charged, or when the charge current has fallen below a prescribed threshold vale, and the battery voltage has continued to exceed a prescribed threshold value for a prescribed time period.